No CPU option in cycles compute device
The feature you're referring to hasn't made it into Blender 2.79b. If I recall correctly, it was finished after the release of 2.79b. If you still want to give it a shot, you can go to the Buildbot page at https://builder.blender.org/download/ and download the Official build from there.

Once you start that one up, the User Preferences will show you these checkboxes when you enable CUDA:



As you can see, my CPU is listed as Intel Xeon CPU E5-2640, and can be used in addition to the TitanX.

On the same computer, Blender 2.79b doesn't show the additional CPU option: